The disclosure concerns a digital printer with at least one print group to print to a recording medium with toner particles that are applied with the aid of a liquid developer, in particular a high-capacity printer to print to web-shaped or sheet-shaped recording media.
In such digital printers, a latent charge image of a charge image carrier is inked by a print group by means of electrophoresis, with the aid of a liquid developer. The toner image that is created in such a manner is transferred to the recording medium indirectly via a transfer element or directly. The liquid developer has toner particles and carrier fluid in a desired ratio. Mineral oil is preferably used as a carrier fluid. In order to provide the toner particles with an electrostatic charge, charge control substances are added to the liquid developer. Further additives are additionally added, for example in order to achieve the desired viscosity or a desired drying behavior of the liquid developer.
Such digital printers have long been known already, for example from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1 or DE 10 2009 060 334 A1.
To ink the charge images on the charge image carrier, liquid developer is directed by a developer station past the charge image carrier. The developer station has in a known manner: a developer roller that directs the liquid developer past the charge image carrier; an application system that supplies the liquid developer to the developer roller; and a cleaning unit that cleans off the residual liquid developer that remains on the developer roller after the inking of the charge images on the charge image carrier.
Developer stations in which liquid developer is supplied to a charge image carrier are known. In U.S. Pat. Nos. 7,522,865 B2, 7,292,810 B2, 6 895 200 B2, developer stations are described in which liquid developer is directed past a developer roller. Arranged adjacent to the developer roller is an electrode between which and the developer roller the liquid developer is directed through. An electrical voltage exists between the electrode and the developer roller, due to which electrical voltage the toner is drawn to the developer roller. Before the liquid developer is supplied to the charge image carrier, it travels through a gap (nip) that exists between a dosing means (for example a blade or a dosing roller) and the developer roller. The dosing means is at such an electrical potential that the toner migrates to the developer roller; and at the same time, the thickness of the liquid developer layer on the developer roller is established. Examples of dosing means are described in WO 2006/090352 A1 and U.S. 2002/0159794 A1.
The developer layer on the developer roller may thus be adjusted by means of the dosing roller in terms of its properties, for example the layer thickness, toner concentration. The two significant influencing variables are thereby the potential difference between the developer roller and the dosing roller and the contact pressure force between the dosing roller and the developer roller or the nip length that results from this. These determine the conveying capacity in the nip, and therefore the toner concentration of the developer layer. It is typical that the two rollers are coupled via a gearing, and that the surfaces of the rollers run synchronously on one another. However, the possibility also exists to affect the composition of the developer layer via a variation of the velocity of the dosing roller given constant velocity of the developer roller.
In order to generate a print image of high quality, it is necessary that the amount of carrier fluid in the liquid developer (in particular the amount of carrier fluid on the recording medium) does not vary unacceptably. Quantities of carrier fluid that are too great upon the transfer of toner image to the recording medium lead to the situation that the print image diverges at the recording medium, and the structures of the print image appear to be blurry and washed-out. It additionally leads to increased costs due to increased consumption of carrier fluid and to a degraded and/or more expensive fixing. The transfer of carrier fluid back into the transfer station may also be negatively affected. Too low a quantity of carrier fluid upon transfer of the toner image to the recording medium has the effect of a poor transfer efficiency of the toner upon transfer printing of the toner image onto the recording medium. Too high a viscosity of the liquid developer may limit the mobility of the toner, and may additionally lead to severe local differences in the transfer efficiency. It is thereby to be considered that the optical quantity of carrier fluid in the transfer of the toner images is also dependent on the material of the recording medium and—given multicolor printing with multiple print group—changes from print group to print group.